As is well known in the art, the burning of solid waste fuels, which are materials with a high moisture and volatile content, is generally accomplished in three overlapping stages, which must be taken into consideration in the design of a suitable furnace for combustion thereof.
The first stage is one for heat absorption in which the moisture in the fuel is evaporated, during which time the temperature of the fuel does not exceed the water boiling temperature at the site of operation.
The second stage is also a heat absorbing stage and thereafter a heat generating stage and occurs when the volatiles are distilled off. In other words, after the evaporation of moisture is completed and with the continued addition of heat, the temperature of the fuel rises. At a temperature of about 600.degree. C. in the presence of air, the volatile matter burns and generates heat.
The rate of moisture evaporation and the distillation of volatiles depend upon the rate with which heat is supplied and also upon the sizing of the fuel. A reasonably small particle size provides a larger surface area and decreases the amount of material through which the heat must be transmitted by conduction.
The third stage occurs when the residual carbon reaches the ignition temperature and burns in the presence of air. Again, in this particular instance, it may be easily seen that the particle sizing of the fuel has a definite influence on the rate of combustion.
As the volatile matter content of these waste or refuse solid fuels is within the range of from 12 to 85% by weight of the total weight of the fuel, it is apparent that the major part of the combustion, which would consume much of the fixed carbon, occurs above the fuel bed, and consequently, the bulk of the combustion air, according to the prior art assumptions, should be supplied in this area.
In order to maintain a suitable environment for a fast evaporation of moisture and distillation of volatiles, it is also apparent that the use of refractory arches and walls is of considerable importance to provide the maximum amount of heat reflected towards the fuel, such that it is shaded from the black or heat-absorbing surfaces.
In order to achieve the above mentioned type of combustion, numerous types of furnaces and burning cells have been used in accordance with the prior art and a historical reviewal of the development of furnaces for solid waste materials, particularly bagasse, has been the subject matter of serious studies and a definite improvement has been arrived at in the design of furnaces and combustion cells therefor.
Thus, before the year 1885, all the solid waste fuels, such as bagasse, were transported from the generation site to fields where the same were sun dried and thereafter were returned to the plant for burning. This rather intricate procedure was required by the bagasse producing mills and the like, because it was required to supply fuel to said mills and the bagasse or similar solid waste materials were considered as a basic heat source. Unfortunately, all the furnaces existing at the time were uncapable of burning such fuels when in the "green" state.
In 1886, the first furnace for burning green bagasse ed and patented by Samuel Fiske. This furnace comprised an oven provided with a horizontal grate built with bars on which the bagasse was burned. The oven was frequently connected to various existing types of boilers by means of flues for combustion gases. The bagasse was fed through feed chutes on the top of the oven, forming two conical piles on the flat grates and refractory materials were installed in all the available areas to provide the maximum amount of radiant and reflected heat for drying and gasifying the volatile materials. This type of ovens for burning green bagasse, while constituting a great advance in the particular art, showed serious drawbacks, inasmuch as on the one hand the ash, in combination with the residual juices in the bagasse, formed a plastic clinker which was very difficult to handle on the flat grates and the use of pre-heated air aggravated the problem, whereby the pre-heating of said air was limited to about 125.degree. C. On the other hand, with a rectangular furnace and conical fuel piles, it is readily apparent that the distribution of fuel on the flat grates was variable and resulted in the formation of "blow holes", wherein no fuel material was arranged on the fuel bed, thereby causing the dilution of the forced draft air which was not contacted with the combustion products, and the flat grate furnaces of Fiske were therefore regarded as highly inefficient.
Later, Frederick Cook introduced a furnace for burning green bagasse comprising a hearth furnace wherein the problems connected with the horizontal grate furnace of Fiske were solved. The hearth was divided into two or more cells in the shape of a horseshoe, whereby these furnaces were named horseshoe furnaces. The cone of fuel material was intimately surrounded by refractory material and, therefore, the efficiency of the furnace was increased. The horseshoe shaped cells were fed through feed chutes at the top of the furnace such as the Fiske burners. The combustion air was forced inwardly and above the fuel cone through tuyeres located in the walls of the cell, from an air plenum which virtually surrounded the lower portion of each cell.
The top of the furnace above the horseshoe cells, had an inclined portion forming a duct which created a turbulence before the combustion gases entered into the chamber of the boiler or similar furnace and, therefore, this increased the efficiency of admixture between the combustion gases and air in order to accomplish a more complete combustion. However, these furnaces, while considerably increasing the efficiency of the furnaces of Fiske, also showed serious drawbacks due to the fact that the volume of the furnace was inadequate and that this type of furnace frequently produced a condition of heavy scale that could not be easily removed from the combustion chambers. Also, all the fine particles of fuel were entrained into the combustion chamber of the boiler and were not generally burned in a complete manner, whereby they adhered in the form of crusts very difficult to be removed on the walls of the boilers and on the tubes thereof, with the consequent disadvantages.
Some time later, the step grate burner or furnace was developed, which comprised a combustion chamber having very steep grates which adopted the form of step ladders, where the combustion air passed through relatively large openings between the steps of the grate in a horizontal direction. This grate suitably supported the fuel and avoided the necessity of small and vertical openings for air as was necessary with the horizontal grate and which frequently were plugged by masses of clinker resulting in an eventual destruction. In this particular instance, even when the possibility of blow holes was minimized, nevertheless said posibility was a potential source of dilution of combustion gases. The bagasse was fed on the upper drying hearth, from which the same was tumbled-down from the upper step of the grate and gradually worked its way to a small bottom flat grate. However, even when this furnace was provided with a roof which was dropped at the outlet in order to cause certain turbulence to achieve better admixture of fuel and air, the problem of practically unburned fine particles of fuel directed towards the combustion chamber of the associated boiler was still present and, consequently, the deposition of said partially burned fine particles on the walls, forming a very difficulty removable crust, remained as a problem even with the above described development. On the other hand, the stepped and inclined grates of this type of combustion chambers were not suitable for fuels with a high moisture content, whereby said furnaces were only of general use in countries wherein the moisture content of fuels was within the lower limits, for instance, Hawaii and the Philippines.
A further development was effected in the year of 1936 by George Ward and was patented by the Babcock & Wilcox Boiler Company. This development comprised a hearth furnace which for the first time constituted an integral part of the installation of a boiler instead of being a separate oven. This was accomplished by increasing the height of the boiler and using the floor space much more efficiently. The furnace comprised one or more cells, very similar to the Cook's cells but with the addition of cast iron hearth plates on which the cell rested. The bagasse was fed to the cells through feed chutes arranged at the front portion of the furnace and the back of the furnace was inclined inwardly thereof, through the use of a refractory suspension in order to again favor the admixing action of gases and fuel and as an aid to effect a combustion as complete as possible, by reflecting the heat again towards the fuel pile and shading the same from black surfaces. In the instance of the Ward furnace, the combustion air was forced inwardly and above the fuel cone through tuyeres located in the walls of the cell and at the cast iron hearth plates, from a plenum which totally surrounded the cell. A dumping hearth was arranged for reducing the labor and for avoiding dilution of cold air in the gases of the boiler and the cooling of refractory materials, whereas the ash was removed manually through a cleansing opening.
While this type of Ward cells constituted a great advance with respect to the fuel combustion technique for solid waste materials, it nevertheless did not accomplish a complete combustion of the fuel and the same problem persisted that the fine particles of said fuels passed in a very green stage towards the combustion chamber of the boiler and, consequently, heavy crusts and scale were formed as described above.
In U.S. Pat. No. 2,602,409 of July 8, 1952, E. L. Dennis described for the first time combustion cells uniformly distributed on the floor of the boiler and also for the first time describes, in said combustion cells, a plurality of tangential tuyeres distributed along the outlet opening of each cell in order to produce a great turbulence in the outlet gases and thus achieve a better combustion. Despite the fact that this was a great achievement with respect to the turbulence forming systems of the prior art burners, the problem caused by the fine particles of fuel that were not sufficiently burned before leaving the cells again persisted and, consequently, the heavy deposits of adherent materials on the walls of the combustion chamber of the associated boiler continued to exist, whereby this type of cells did not solve the above described particular problem.
In U.S. Pat. No. 2,754,778 of July 17, 1956, E. L. Dennis describes certain improvements effected in the burner cells of U.S. Pat. No. 2,602,409, with an attempt to improve the efficiency of combustion of the fine particles, such as pith, by the installation of two lines of tangential tuyeres, one directly above the radial tuyeres to burn the fuel pile and the other one situated, as in U.S. Pat. No. 2,602,409, in the throat of the cells. With the above, a higher turbulence was achieved which obviously tended to burn with a higher efficiency both the volatile materials and the fines or pith, which according to E. L. Dennis should be totally burned in the combustion chamber of the boiler which was also of an increased height in order to avoid the pith to reach the tubes or the walls of the boiler. However, while this new development avoided in a certain degree the incomplete combustion of fines from the solid fuel, as well as it removed the majority of the fines from the larger particles which remained in the burning pile, it nevertheless did not solve the above mentioned problem that, being the partially burned fuel material highly adherent, the same adhered to the walls both of the cells and the combustion chamber of the associated boiler, thus causing very heavy crusts which had to be removed frequently, with the consequent long and uneconomical stops of the furnace.
Finally, in Mexican Pat. No. 76,602, filed Feb. 15, 1963, Edwin L. Dennis describes a further improvement with respect to the burner cells of U.S. Pat. Nos. 2,602,409 and 2,754,778 mentioned above, which improvements reside only and exclusively in providing an additional throat having dimensions slightly smaller than the dimensions of the diameter of the chamber of the cell and in providing under each said throat, that is, the outlet throat of the cell and the additional throat, the above mentioned tangential tuyeres in order to cause a rotatory motion of the outlet gases which entrained the fines. However, this improvement did not achieve any result that may be regarded as different from those accomplished by the arrangement of U.S. Pat. No. 2,754,778, inasmuch as the provision of an intermediate throat has nothing to do whatsoever with the motion of the gases, which is caused exclusively by the provision of the tangential tuyeres.
In all the prior art apparatus, as it may now be clearly apparent, there is the serious problem that the combustion of the fines is absolutely incomplete and that the partially burned fines tend to adhere to the walls of the boilers and to the water tubes thereof, as well as to the walls of the burning cells, thereby considerably decreasing the efficiency thereof. The tangential air streams that were introduced by Dennis, do not solve the problem but, on the contrary, tend to aggravate the same, inasmuch as, when the gases are provided with a rotatory motion, the particles tend to be centrifugally displaced towards the periphery and adhere with more strength on the walls of the chamber, which are thereby considerably affected and must be cleansed very frequently. This has been a continuous and unsolved problem up to the present date in the prior art existing cells for burning solid waste fuel materials.